Why is my iphone turning music on/off and dropping calls when I move it?

I have to stand really still if I want to listen to my music or talk to someone because the phone will act up. If I move my phone to fast or bump it gently while music is playing it would turn on/off or would speed up really fast. The song would pick up where it left off once I hit the unpaused button.The same day I noticed this problem was the same day when I placed my phone into my back pocket so I'm guessing that had something to do with it. I just want to know if there is a specific reasons like what is damage, lose,... things of that nature. My phone is unlock and I do not have a warranty on it any more. I can get it fixed I just want to know whats wrong it